Diffusion bonding involves the molecular interdiffusion of alloy components between two contacted bodies resulting in a metallurgical bond. Generally, diffusion bonding is a solid state process which avoids substantial liquification of the contacted bodies during joining. Various methods exist which promote such interdiffusion. One method involves an intermediate bonding material mutually contacting the two articles to be joined. Such methods are disclosed in U.S. Pat. Nos. 3,632,319 to Hoppin, et al and 4,412,643 to Sato, et al. Bonding occurs without the application of external pressure and generally relies on the presence of a melting point depressent in the bonding material which provides a liquidus temperature less than the incipient melting temperatures of the contacted bodies. As the melting point depressant diffuses away from the bonding site, solidification occurs, joining the two bodies together. Another method utilizes hot isostatic pressing (HIP), which requires a combined application of temperature and pressure to promote solid state diffusion. Such a method is disclosed, for example, in U.S. Pat. No. 3,940,268 to Catlin, where rotor disks are bonded to a hub at temperatures of 1500.degree.-2400.degree. F. and pressures of 300 to 60,000 PSI.
While such diffusion bonding processes are generally applicable, there are several limitations. For a process relying on an intermediate bonding material, an additional step is required in the bonding process involving the application of either a coating or a powder to one or both of the bodies. Generally, the composition of the intermediate bonding powder must essentially match the components of the bodies to be bonded, limiting this process to joining bodies of similar composition. Utilizing a hot isostatic process can achieve a metallurgical bond without the use of an intermediate bonding material. However, this process generally requires very large and expensive equipment to provide the uniform temperatures and high pressures needed to achieve a metallurgical bond, with the size of the equipment increasing with the size of the bodies to be joined. Consequently, hot isostatic pressing is economically limited to bonding small bodies.
In U.S. Pat. No. 4,386,959 to Frehn, a method is disclosed for forming a connection between a sintered pressed powder metallurgy body and a metallic compact body. This method relies on the inclusion of at least one powder component in the powder metallurgy body which forms a liquid phase at the sintering temperature. Direct contact between the powder metallurgy body and the compact metallic body is required. While such a method is useful in certain applications, the presence of a liquid phase in a powder metallurgy body during sintering could significantly alter the final alloy properties. In particular, thermal resistance would be greatly reduced.
Alternatives to diffusion bonding include brazing and shrink fitting. These, in turn, have other processing limitations. While brazing can provide a metallurgical bond, the diffusion of undesirable components such as flux materials from the brazing composition into the adjoining bodies may weaken the surrounding alloys or cause grain boundary attack. Shrink fitting, which relies on tight physical contact to frictionally hold one body to another, does not provide a metallurgical bond and, therefore, is not as strong as either brazing or diffusion bonding. Consequently, shrink fitting is unacceptable in many applications.